FIELD OF THE INVENTION
The present invention relates to a process for the preparation of polycarbonates, more specifically to a process for the preparation of polycarbonates through solid-state polymerization of a prepolymer from aromatic dihydroxides and diarylcarbonates, which comprises
a) preparing a prepolymer having a viscosity average molecular weight from 4,000 to 18,000 g/mole; PA1 b) preparing microporous prepolymer foams by introducing a high-pressure inert gas; and PA1 c) preparing polycarbonates by a solid-state polymerization, PA1 a) preparing a prepolymer having a viscosity average molecular weight of 4,000-18,000 g/mole by the melt polymerization of aromatic dihydroxides and diarylcarbonates; PA1 b) preparing a prepolymer foam having a specific gravity of 0.5-1.15 by introducing high-pressure inert gas, stirring and then draining the prepolymer; PA1 c) preparing a solid particle with a crystallinity of 10-50% by crushing the prepolymer foam to the average size of 0.1-2 mm and stirring in the solvent; and PA1 d) preparing polycarbonates by performing a solid-state polymerization of the solid particles at a temperature of 190-240. PA1 a) Ar.sub.1 and Ar.sub.2 represent the same or different phenyl group or its derivatives; and PA1 b) Z represents a single bond or --O--, --CO--, --S--, --SO.sub.2 --, --SO--, --CON(R.sub.1)-- or --C(R.sub.2 R.sub.3)-- linkage; R.sub.1, R.sub.2 and R.sub.3 repsectively represents H or --(CH.sub.2).sub.n CH.sub.3 ; and n is an integer in the range of 0-4.
which facilitates the removal of phenol, a by-product, and enhances molecular weight of polycarbonates without using any catalyst due to the relatively high polymerization.
Polycarbonates are widely used in the manufacture of transparent sheets, packaging materials, bumpers for automobiles and compact discs due to its superior transparency, impact resistance, mechanical strength and heat resistance.
Polycarbonates are generally prepared by an interfacial polymerization, where polycarbonates are generated by vigorously mixing aqueous bisphenol-A solution substituted with sodium with a phosgene-containing organic solution. However, this method has the drawbacks that the phosgene used as a starting material is a toxic gas and the solvent used in the polymerization is a chlorine-containing organic solvent that can pollute the air. Moreover, the excessive amount of water used in the production of polycarbonates even after the polymerization requires a further purification of said polycarbonates. A melt polymerization method, which prepares polycarbonates by direct polymerization of monomers without using solvent, has been introduced to solve the above problems. However, the melt polymerization method is also disadvantageous in that it requires not only such a reaction condition of high temperature and high vacuum to remove phenol, by-product but special apparatus also to stir high viscose reaction mixture to obtain the high molecular weight polycarbonates.
A solid-state polymerization method was disclosed in U.S. Pat. Nos. 5,266,659 and 5,717,056 as a way to remedy the melt polymerization method, where a melt polymerization is terminated at a state with a low viscosity to generate solid particles and said particles are allowed to undergo a polymerization while maintaining the solid state. However, the solid-state polymerization method, which requires a reaction between solid-state prepolymers with high molecular weight as well as the easy removal of a reaction by-product, does not appear plausible because its low polymerization rate results in a poor productivity. A catalyst can be added to increase the above polymerization rate in the solid-state polymerization; however, the method using a catalyst is not only costly but can also deteriorate the properties of polycarbonates due to the residues of said catalyst in the final polymers. U.S. Pat. Nos. 4,948,871 and 5,204,377 disclosed the processes of preparing polycarbonates without using a catalyst by a solid-state polymerization at a temperature over 200 by preparing prepolymer from bisphenol-A and diphenylcarbonate, crystallizing in solvents such as acetone and toluene and maintaining the prepolymer in a solid-state to facilitate the easier remove of phenol, a reaction by-product. However, this method also has a low rate of the reaction.
As shown above, the conventional preparation methods of polycarbonates have been shown to have a rather slow reaction rate and be not suitable for the easier removal of the by-product, thus necessitating an urgent emergence of a new version of polycarbonate preparation method.